1. Field of the Invention
The present invention relates to a record sheet for use in an electro-coagulation printing method by forming an ink layer representing an image of desired characters and pictures on a cylinder which constitutes an electrode and transferring this ink layer to the record sheet under pressure and in particular, to a record sheet having an excellent dot reproduction and tone reproduction of printed images.
2. Prior Art
The electro-coagulation printing method has been well known as disclosed in, for example, U.S. Pat. No. 3,892,645, No. 4,555,320 and No. 4,764,264, and JPA Hei 4-504688. An ink used in the electro-coagulation printing method is water ink. The water ink is composed of water, a polymer which electrolytically coagulates, a soluble electrolyte, and coloring agent. Examples of the polymer which electrolytically coagulates are albumin, gelatine, casein, agar, polyacrylate, polyacrylamide, and PVA. Examples of the soluble electrolyte are lithium chloride, sodium chloride, calcium chloride, potassium chloride, nickel chloride, copper chloride, and magnesium sulfate.
The electro-coagulation process is basically performed in the following manner. In the state that the above-described ink layer is present between a positive electrode and an negative electrode, when an electric potential is produced therebetween, colloid coagulates and adheres to the positive electrode. The coagulation takes place in the state that the colloid is colored with a coloring agent contained in the ink. By arranging the colored coagulated colloid in a pattern corresponding to a desired image, the desired image can be reproduced. By transferring the reproduced image to a record sheet by a proper method, the desired image is recorded on the record sheet.
The structure of a printer according to the electro-congulation printing method is described in the above-mentioned JPA Hei 4-504688. Referring to FIG. 1, the structure of the main part of a conventional printer according to the electro-coagulation printing method will be described in brief. FIG. 1 is a schematic diagram showing a structure of a printer for forming an image of a monochrome picture and transferring the image to a record sheet. When an image with a multiple colors is printed, a desired number of the same units are used corresponding to the number of the desired colors. In FIG. 1, reference numeral 1 depicts a metal cylinder which functions as a positive electrode. The metal cylinder is composed of a metal which is electrically inactive such as stainless steel. Two cylindrical electrodes 2 are independently disposed on the periphery of the positive electrode 1. The cylindrical electrodes 2 are insulated from the electrode 1. An amount of ink sprayed from an ink spraying device 3 is filled in a nip between the electrodes 1 and 2. The positive electrode 1 is continuously rotated in the clockwise direction in FIG. 1. With a potential difference between the positive electrode 1 and the negative electrodes 2, coagulated colloid portions and non-coagulated portions are formed in the ink filled between the positive electrode 1 and the negative electrodes 2. The coagulated colloid adheres to the positive electrode 1. Only the non-coagulated portion is selectively removed from the positive electrode by a wiper 4 or the like.
A press roll 5 is pressed against the periphery of the positive electrode 1. A record sheet 6 is traveled by the positive electrode 1 and the press roll 5. Thus, the coagulated colloid held on the periphery of the press roll 5 is placed in the position of the press roll 5 as the positive electrode 1 rotates. The coagulated colloid is contacted and transferred to the record sheet 6. At this point, the nip pressure between the press roll 5 and the positive electrode 1 is in the range from 30 to 50 kg/cm. After the coagulated colloid has been transferred to the record sheet, the positive electrode 1 is further rotated, and then cleaned by a cleaning device 7. Thereafter, a corrosion resisting agent is coated on the periphery of the positive electrode by a corrosion resisting agent coating device 8. Thus, one cycle of the printing process has been completed.
When compared with the conventional printing methods such as offset printing method, letterpress printing method, screen printing method, and gravure printing method, as a major difference, the electro-coagulation printing method is categorized as so-called "non-plate printing method". The non-plate printing method has many advantages over the conventional printing methods. In the conventional printing methods, a step for forming a printing plate is essential. The cost for the printing plate per one print sheet is usually very large. On the other hand, since the non-plate printing method does not need the printing plate forming step, the cost is greatly reduced. In addition, in the case of the conventional "plate printing method", although the step for printing the same prints can be performed at high speed, it takes a long time to replace the plates. In contrast, in the "non-plate printing method", data is received from a computer is read and printed. Thus, the preparing time for different prints is very shoot. Consequently, it can be considered that the electro-coagulation printing method is much superior to the conventional printing methods particularly in a small lot printing.
In addition, since the preparing time for printing different prints is very short, so-called page variable process where the base text of direct mails and individual addresses are printed, which is impossible in the conventional printing methods, can be performed.
Moreover, the printer using the electro-coagulation printing method is composed of relatively rigid and simple parts. Thus, the printer can be operated at high speed. The upper limit of the printing speed depends on the information transmission speed of the computer rather than the printer. With a conventional computer, the printing speed at a level of several hundred meters per minute can be satisfactorily accomplished. The printer is operated at a high speed and has a high productivity as compared with recent printers for a rapidly progressing "non-plate printing".
The coloring agents used in the electro-coagulation printing method may be the same as those used for inks in the conventional printing methods. The shape and size of the coagulated colloid in the electro-coagulation printing method are almost the same as those of the negative electrodes. In the electro-coagulation printing method, a so-called "dot gain" phenomenon does not take place on the record sheet. Thus, an image can be clearly reproduced with fine and sharp dots.
As described above, it is considered that the electro-coagulation printing method is an excellent printing method featuring high through-put and high picture quality available in the conventional printing methods. In addition, the electro-coagulation printing method has also features which are a small lot printing and a page variable property that were not available by the conventional printing methods.
The present inventors filed Japanese Patent Application No. 319038/1996 (JPA 10-131091 Laid-open on May 19, 1998) as a earlier application. This reference discloses a record sheet for an electro-coagulation printing method having such properties that a wet time and absorption coefficient obtained from a liquid absorption curve of pure water measured by a dynamic scanning absorptometer is not more than 15 milliseconds and is at least 5 ml/m.sup.2 s.sup.-1/2, respectively in case of requiring the quality of a fine monochrome image and also, that a wet time and absorption coefficient obtained from a liquid absorption curve of pure water measured by a dynamic scanning absorptometer is not more than 15 milliseconds and is at least 10 ml/m.sup.2 s.sup.-1/2, respectively and a contact ratio measured by a specular reflection smoothness tester under a pressure of 40 kg/m.sup.2 with a ray having a wavelength of 0.5 .mu.m is at least 40% in case of requiring the quality of a full-color image.
As mentioned above, since the electro-coagulation (hereinafter referred to as "the Elco") printing method is very excellent, when the record sheet disclosed in the above earlier application is used, a good image-reproduction is obtained. On the other hand, there have been required a record sheet improved in a dot reproduction and tone reproduction.